Technical Field
Embodiments described herein relate to a technique for connecting multi-layer wire pieces each of which is formed of a laminated thin layer and formed into a tape shape.
Description of the Related Art
In recent years, the study of a high-temperature superconducting coil, using a high-temperature superconducting wire represented by a REBCO wire using (RE)Ba2Cu3O7 including rare earth (RE), has been actively conducted.
Especially, a high-temperature superconducting wire (hereinafter referred to as “multi-layer wire”), manufactured by composing a plurality of types of layers on a substrate having a thickness of about 100 μm, has the characteristic of having a large current capacity under a high magnetic field.
The multi-layer wire has the characteristic of not losing superconducting properties even when receiving high stress in the tape longitudinal direction.
It is expected to realize a high-temperature superconducting coil which allows high stress and high current density required to generate a high magnetic field.
When the multi-layer wire is applied for a MRI magnet, a magnet for a single crystal pulling apparatus, an accelerator magnet, and the like, the wire length required for the magnet becomes several [km] to several hundreds of km.
Since a high-temperature superconducting layer requires high crystallinity, it is difficult to obtain a long multi-layer wire. Under the present circumstances, the length of a single multi-layer wire ranges from several tens [m] to several hundred [m].
Pluralities of the pieces are needed when a high-temperature superconducting coil is manufactured by connecting the multi-layer wire pieces.
At present, many kinds of efforts is made for the multi-layer wire pieces have been used to reduce the connection resistance as much as possible, since a method for zero-resistance connection, so-called superconducting connection, has not been established.
Each of top and bottom layers constituting the layers of the multi-layer wire is usually a stabilizing layer formed by plating, such as copper plating.
A superconducting current flowing through the high-temperature superconducting layer more easily flows into the stabilizing layer located on the same side as the superconducting layer with respect to the substrate, where the side of this stabilizing layer is hereinafter referred to as “obverse”.
The obverses of them are bonded to reduce resistance when multi-layer wires are extended by being connected to each other by solder.
Similarly, when the multi-layer wire is connected to a normal conductor, such as an electrode, the connection resistance is reduced by connecting the obverse to the normal conductor.
It should be noted that the heat which is generated when the superconducting current flows through the normal conductor such as solder has almost no effect on the superconductivity of the high-temperature superconducting wire.
That is, the density of the heat which the normal conductor generates is low, and hence, when the superconducting coil is sufficiently cooled, the function of the superconducting coil is not lost.
However, when the superconducting layer is destroyed, the density of the heat generated at the destroyed portion is significantly increased.
It is difficult to keep the destroyed portion at a low temperature by cooling in a refrigerator or the like, and hence it is considered that the thermal runaway occurs. For example, see Patent Documents: Japanese Patent Applied-Open No. 2000-133067, No. 2008-140930 and No. 2011-018536.
As described above, as for the superconducting properties, the multi-layer wire has an advantage of having a high allowable stress in the so-called tape longitudinal direction.
However, it is confirmed that, when the multi-layer wire is connected by soldered on the obverse, the allowable stress in the tape longitudinal direction is reduced.
It is found that the reduction of the allowable stress is due to the concentration of stress at the connection end portion.
The multi-layer wire generally has a high allowable stress in the tape longitudinal direction, but on the other hand, has a low allowable stress in the direction in which the layers composing the multi-layer wire are peeled off (hereinafter referred to as “peeling direction”).
The reduction of the allowable stress in the tape longitudinal direction is due to the fact that, when a pulling force is applied in the tape longitudinal direction, stress is concentrated at small portions at both ends of the connecting portion.
It is expected that a part of the concentrated stress becomes a stress component in the peeling direction (hereinafter referred to as “peeling stress”), which destroys the laminated body including the superconducting layer.
As described above, the destruction of the laminated body may lead to thermal runaway.
The high-temperature superconducting coil composed of the multi-layer wire pieces usually cannot perform the function as the superconducting coil, when, even in a part of the multi-layer wire pieces, the superconductivity is lost due to the thermal runaway or the like.
On the other hand, in the case where the outer periphery of the multi-layer wire is reinforced with a high-strength and high-electrical-resistance material, the allowable stress of the thin layer in the tape longitudinal direction is not reduced even when the multi-layer wire pieces are connected to each other.
This is because, since the peeling stress generated at both ends of the connecting portion is imposed to the reinforcing material, the superconducting layer in the multi-layer wire is hardly affected by the peeling stress.
Once the outer periphery of the multi-layer wire is reinforced, however, the cross-sectional area of the multi-layer wire with respect to the current-carrying capacity is increased, and thereby the current density of the high-temperature superconducting coil as a whole is reduced.
The reduction of the current density is disadvantageous from the viewpoint of effectively generating a high magnetic field.